1. Field of the Invention
The present invention generally relates to semiconductor devices, and more particularly to a BiFET semiconductor device having vertically integrated field effect transistors (FET) and hetero-junction bipolar transistors (HBT) on the same substrate.
2. The Prior Arts
Integrating a FET and a HBT on the same substrate is commonly referred to as a BiFET semiconductor device. The benefit of BiFET devices is well known in analog circuits as they are operable under very high frequency while offering greater functionality. These features are particularly useful in RF or mobile communication devices. One typical application of the BiFET device is in implementing the power amplifier (PA) in a cellular handset.
For conventional BiFETs, FETs and HBTs are integrated laterally on the same substrate. One such lateral integration of FETs and HBTs is disclosed in U.S. Pat. No. 5,280,826, whose reference drawing is included as FIG. 1.
As illustrated, a laterally integrated HBT-FET device 10 contains an n-p-n GaAs HBT 11 formed on the substrate 12 and the HBT 11 includes, from bottom to top, a n+ GaAs layer 14 as sub-collector, a n− GaAs layer 16 as collector, a p+ GaAs layer 18 as base, and an emitter structure, which is composed of a n− AlGaAs layer 20 as emitter, a n− GaAs layer 22 as emitter cap, and a n+ InGaAs layer 24 as emitter contact. A metal semiconductor FET (MESFET) 15 is constructed using the same layers of material of the HBT 11's emitter structure. The n-FET 15 has a source (S) and a drain (D) formed in the n+ InGaAs layer 24, a gate recess etched in the InGaAs layer 24 between the source and drain, and a Schottky gate metal contact (G) deposited on the n− GaAs layer 22 exposed in the gate recess. The n-p-n HBT 11 and the n-FET 15 are isolated by ion implantation 26.
Enabling greater functionality in a smaller package is a key challenge for MMIC (monolithic microwave IC) manufacturers. In the lateral integration of FETs and HBTs, the FETs constructed are usually MESFETs whose performance is not as good as Pseudomorphic High Electron Mobility Transistors (PHEMT). If the vertical integration of HBTs and FETs could really be achieved, more advanced PHEMT could be adopted, giving the BiFET device more flexibility in terms of its application and performance. However, to construct BiFET semiconductor device having a vertical structure requires effective isolation of the vertically stacked FETs and HBTs, which is very difficult.
The technique provided by Hata et al. in U.S. Pat. No. 5,332,451 suggests a way to achieve such an effective isolation. The objective of Hata et al. is to provide an epitaxial crystal with high resistivity to enable the fabrication of high-speed electronic elements. The epitaxial crystal provided by Hata et al. comprises a substrate, a buffer layer, and an active layer sequentially formed in this order from bottom to top. The buffer layer comprises, from bottom to top, a first layer made of AlGaAs, AlGaInP, or InAlAs doped with oxygen and/or a transition metal, and a second layer made of high-purity GaAs, InGaP, AlGaAs, or InP. The dopant oxygen or transition metal in the first layer is for achieving high resistivity, and the second layer is constructed so that the dopant's influence on the electronic elements (such as FET) fabricated on top of the epitaxial crystal could be diminished to a practically negligible level.